1. Field of the Invention
The present invention relates to a method for producing a joined body of AlN substrate and a joining agent used for the joining.
2. Related Art Statement
Heretofore, in conventional semiconductor production apparatuses, such as, etching apparatuses, CVD apparatuses and the like, so-called stainless heaters and indirect heating system heaters are generally used. However, when these heating sources are used, there are problems in that they are likely corroded by halogen-series corrosive gases to form particles thereon and that they have inferior heat efficiency. In order to solve such problems, the applicant previously disclosed in his Japanese Patent Application Laid-Open No. 3-261,131 a ceramic heater having a heating wire of high melting point embedded in a dense ceramic substrate, the wire being spirally wound in the interior of the substrate of a disc shape and connected at the both ends with an electric terminal, respectively. Such a type of ceramic heaters are found to have superior characteristic properties, particularly for the production of semiconductors.
As the ceramic materials constituting the substrate of the ceramics heaters, silicon nitride, aluminum nitride, Sialon and the like nitride-series ceramic materials are considered preferable. In some cases, a susceptor is provided on the ceramic heater and a semiconductor wafer is mounted and heated on the susceptor. The applicant previously disclosed in his Japanese Patent Application Laid-Open No. 5-101,871 that aluminum nitride is preferable as the substrate for such ceramic heaters and susceptors. This is because, particularly in semiconductor production apparatuses, ClF3 and the like halogen-series corrosive gases are often used as etching gases or cleaning gases, and aluminum nitride was found to have an excellent corrosion resistant property to such halogen-series corrosive gases. Meanwhile, because ceramic materials are difficult to process, researches have been made of joining ceramic materials of simple shapes to each other to obtain a ceramic element or part of complicated shapes.
Nevertheless, at a joining interface between ceramic members a third phase having a different thermal expansion coefficient and mechanical properties is usually formed. The third layer has a problem in that it is usually easily broken by a thermal stress due to heating and cooling and various mechanical stresses. Particularly, in the case of aluminum nitride-series ceramics materials, the influence of the third layer was serious, because of their low tenacity as compared with silicon nitride-series ceramic materials, etc.
If aluminum nitride-series ceramic materials are joined to each other by means of a glass or a compound consisting mainly of silicon, the third phase remaining on the joining interface is selectively corroded by a plasma of NF3, ClF3 or the like halogen series-corrosive gases. Thus, such joined bodies could not withstand the use under the corrosive environment of the semiconductor production apparatuses.
There is also a method of directly joining the substrates made of aluminum nitride sintered bodies to each other as described in Japanese Patent Application Laid-Open No. 2-124,778, wherein the substrates are heated at 1,800-1,900xc2x0 C. and joined integrally by diffusion joining. However, in order to join the aluminum nitride sintered bodies by such a diffusion joining method, an extremely high temperature is necessary of, for example, 1,800-1,900xc2x0 C. which is substantially the same high temperature with the sintering temperature for producing the original aluminum nitride sintered bodies. Henceforth, in the joining process the substrates are likely degraded and deformed. In addition, joined bodies of a low strength of not more than about 60 MPa could only be obtained.
According to Japanese Patent Application Laid-Open No. 8-13,280, a joined body of aluminum nitride sintered bodies is disclosed having a relatively high strength. However, in this method also, a substantially the same high temperature with the sintering temperature for producing the aluminum nitride sintered bodies of the original substrates is required. In addition, super precise processing of the joining surfaces of the substrates of a roughness and a flatness respectively of not more than 0.2 xcexcm is required. Such a super precise processing increases the production cost.
An object of the present invention is to provide a novel method of producing a joined body of aluminum nitride-series substrates to each other in the substantial absence of an intervening third layer at the joining interface of the substrates.
The present invention is a method of producing a joined body of aluminum nitride-series substrates, comprising a first step of interposing a joining agent between the substrates to be joined, and heating the substrates and the joining agent at a temperature range of not less than the melting point of the joining agent to thereby melt the joining agent and liquefying particles of the aluminum nitride-series substrates to a liquid phase adjacent the interfaces between the melted joining agent and the substrates, and a second step of heating the substrates and the joining agent at a temperature range of higher than the temperature range of the first step, but lower than the sintering temperature of the substrates to thereby exhaust the joining agent from between the substrates.
The inventors have made many researches on methods of joining the aluminum nitride-series ceramic materials at relatively low temperatures without interposing a third phase. For that purpose, the inventors have made many experiments of interposing various metal oxides between the aluminum nitride-series ceramic materials and heating and melting the metal oxides. As a result, the inventors have found out that when the joining agent is at first melted and then heated and held at a temperature higher than the melted temperature, the joining agent is exhausted from between the substrates thereby to form a firm joining between the substrates. Observation of the interface of these substrates revealed that the joining agent is substantially not remained to leave a continuous texture of aluminum nitride substrates. In addition, the inventors have found out a finding leading to the present invention that the thus obtained joined body has very a high joining strength and much superior airtight property and corrosion resistant property.